**4.2 Temperature**

156 Biogas

(HRT). The advantage is that the digester can operate with a high flow and can tolerate higher concentrations of toxic species than in (CSTR) systems. The biofilm system operates normally in a continuous mode with an (HRT) lower than 5 days. The systems can operate in a wide range of temperature and psychrophils conditions (3°C) up to the extrathermophiles conditions (80°C). For the anaerobic treatment of soluble organic waste the

In CSTR systems, the biomass is suspended in the main liquid and will be removed as well as the effluent so that the solid retention time (SRT) is equal to the hydraulic retention time (HRT). This makes it necessary to operate at a high hydraulic retention time (HRT) , between 10 and 20 days, to avoid the scrubbing of the methanogens which have a long time

The factors affecting the production of biogas are mainly based on the operating conditions of the digester, such as pH and temperature which influence directly the micro-organisms. The perturbations in effluent (including the concentration of substrate and its composition in toxic compounds and inhibitors) can also affect the volume and the quality of the produced biogas. Sometimes, the toxic compounds are not present at the beginning in the effluent waste, but they are produced inside the digester starting from degradation of

The type and the composition of the substrate determine directly the quality of the produced biogas. In anaerobic process the substrate is often measured in term of chemical oxygen demand (COD) or of total volatile solids (TVS). It is significant to distinguish between the degradable and the inert fraction, because a considerable fraction of the COD in effluent is inert (Nielsen, 2006). The waste which contains a high percentage of water has a

Organic waste contains various compounds: mainly saccharides (which are divided into two fractions, easily and slowly degradables), lipids (easily degradable), proteins (easily degradable), VFA (easily degradable), as well as others compounds (Moosbrugger & al., 1993). The production of methane is generally in the range from 100 to 400 L CH4 / kg VS. On the other hand, the majority of organic waste contain a high fraction of the substrate easily degradable, which gives a high production of methane and VFA. It is thus significant to control the organic and hydraulic loading according to the capacity of the digester when the process functions are at low charge that gives also a low production rate of biogas. Although this can prevent the rupture of the process, it is not very ecomical because the capacity of the process is not completly used. The increase in the charge gives more biogas but also there is the risk of the overload, with, as a consequence, the accumulation of the VFA. The high concentration of VFA decreases the pH and making them more toxic for

Sufficient nutriments are also impotant for microbial growth. The macro nutriments such as carbon, hydrogen, nitrogen and oxygen are the main components of the cells in the biomass,

**4. Factors affecting the anaerobic digestion process stability** 

systems of UASB at high rate are used.

substrate (example: VFA and ammonia).

weak methane yield by COD or VS.

methanogens bacteria.

of growth.

**4.1 Substrate** 

Anaerobic digestion can be applied in a wide range of temperature, into psychrophilous (< 20 °C) (Vavilin & Angelidaki, 2005), in mesophile (25-40 °C), thermophile (45-60 °C) (Angelidaki & al., 2005), and even in extra thermophile conditions (> 60 °C) (Liu, 2003). The temperature has a direct effect on the physicochemical properties of all the components in the digester and affects also the thermodynamics and the kinetics of the biological processes. The temperature determines if a specific reaction is favorable.

The increase in the temperature has the following advantages:

